1. Field of the Invention
This application relates generally to the deposition of silicon-containing materials, and more particularly to chemical vapor deposition of silicon-containing films over mixed substrates.
2. Description of the Related Art
A variety of methods are used in the semiconductor manufacturing industry to deposit materials onto surfaces. For example, one of the most widely used methods is chemical vapor deposition (“CVD”), in which atoms or molecules contained in a vapor deposit on a surface and build up to form a film. Deposition of silicon-containing (“Si-containing”) materials using conventional silicon sources and deposition methods is believed to proceed in several distinct stages, see Peter Van Zant, “Microchip Fabrication,” 4th Ed., McGraw Hill, New York, (2000), pp. 364-365. Nucleation, the first stage, is very important and is greatly affected by the nature and quality of the substrate surface. Nucleation occurs as the first few atoms or molecules deposit onto the surface and form nuclei. During the second stage, the isolated nuclei form small islands that grow into larger islands. In the third stage, the growing islands begin coalescing into a continuous film. At this point, the film typically has a thickness of a few hundred angstroms and is known as a “transition” film. It generally has chemical and physical properties that are different from the thicker bulk film that begins to grow after the transition film is formed.
Deposition processes are usually designed to produce a particular type of bulk film morphology, e.g., epitaxial, polycrystalline or amorphous. When using conventional silicon sources and deposition processes, nucleation is very important and critically dependent on substrate quality. For example, attempting to grow a single-crystal film on a wafer with islands of unremoved oxide will result in regions of polysilicon in the bulk film. Because of these nucleation issues, deposition of thin film Si-containing materials with similar physical properties onto substrates having two or more different types of surfaces using conventional silicon sources and deposition methods is often problematic.
For example, silicon tetrachloride (SiCl4), silane (SiH4), and dichlorosilane (SiH2Cl2) are the most widely used silicon sources in the semiconductor manufacturing industry for depositing Si-containing films, see Peter Van Zant, “Microchip Fabrication,” 4th Ed., McGraw Hill, New York, (2000), p 380-382. However, deposition using these conventional silicon sources is generally difficult to control over mixed substrates, such as surfaces containing both single crystal silicon and silicon dioxide. Control is difficult because the morphology and thickness of the resulting Si-containing film depend on both the deposition temperature and the morphology of the underlying substrate. Other deposition parameters, including total reactor pressure, reactant partial pressure and reactant flow rate can also strongly influence the quality of depositions over mixed substrates.
For example, FIG. 1A schematically illustrates a cross-section of a substrate 100 having an exposed silicon dioxide (“oxide”) surface 110 and an exposed single crystal silicon surface 120. FIGS. 1B and 1C schematically illustrate the results obtained by using silane in a chemical vapor deposition process to deposit a silicon film onto the substrate 100. For temperatures of about 625° C. and below, deposition conditions can be selected that result in a low defectivity, epitaxial silicon film 130 over the epitaxial surface 120, but under such conditions no film (FIG. 1B) or a film 140 having poor quality (FIG. 1C) is deposited over the oxide surface 110. The differences in film formation are believed to be a result of the differences in nucleation rates on the two surfaces when silane is used as the silicon source. Conventional silicon precursors demonstrate well-documented poor nucleation over dielectrics, such as silicon oxide. By the time spotty nucleation sites converge on the oxide, deposition over adjacent non-dielectric regions has progressed considerably. Furthermore, deposition tends to be rough over the dielectric since widely spread nucleation sites support deposition while regions between remain bare. Often, the illustrated “selective” epitaxial deposition is desired (FIG. 1B); in other cases, better deposition of silicon over the oxide surface 110 is desired, e.g., to facilitate later contact to the epitaxial region.
In theory, the deposition parameters could be adjusted to improve the film formation over the oxide surface, but in practice this is rarely an option because such an adjustment would be likely to negatively impact the desired epitaxial film quality. In many cases, the desired performance characteristics of the resulting semiconductor device dictate the thickness, morphology, temperature of deposition and allowable deposition rate of the Si-containing film that is deposited over the epitaxial surface. The needed thickness and morphology, in turn, dictate the deposition conditions for the film. This is especially the case for heteroepitaxial films that are strained on single crystal silicon substrates. Therefore, the manufacturer generally has little freedom to adjust the conditions to alter the characteristics of the film over the oxide surface. Similar problems are also encountered in situations involving other mixed substrates.
In the past, manufacturers have approached such problems through the use of selective deposition or additional masking and/or process steps. For example, U.S. Pat. No. 6,235,568 notes that one is presently unable to selectively deposit a silicon film onto p-type and n-type silicon surfaces at the same time. U.S. Pat. No. 6,235,568 purports to provide a solution to this problem by carrying out a pre-deposition low energy blanket ion implantation step. The stated purpose of this additional step is to make the surfaces appear the same to a subsequent deposition process.
However, additional process steps are generally undesirable because they may increase expense, contamination and/or complication. The ability to deposit satisfactory mixed morphology Si-containing films over mixed substrates would satisfy a long-felt need and represent a significant advance in the art of semiconductor manufacturing.